This invention relates to a gear-type pump which pressurizes a fluid by the movement of pump chambers and their volume change which are caused by the rotation of an inner rotor and an outer rotor, and more particularly to such a gear-type pump best suited for a fuel pump of an automobile.
Conventional gear-type pumps are known, for example, from Japanese Patent Unexamined Publication Nos. 63-223382 and 60-17281. Such a gear-type pump comprises a rotor housing, and a pair of rotors mounted within the rotor housing. The number of teeth of the inner rotor is smaller by one than the number of teeth of the outer rotor. The inner rotor is driven and rotated to rotate the outer rotor, meshed with the inner rotor, in the same direction as the direction of rotation of the inner rotor. By doing so, a plurality of pump chambers formed between the inner and outer rotors move in such a manner that the volume of the pump chamber is changed. A fluid is drawn from an intake port provided at a region where the volume of the pump chamber is gradually increasing, and the fluid is discharged from a discharge port provided at a region where the volume of the pump chamber is gradually decreasing.
Particularly, Japanese Patent Unexamined Publication No. 60-17281 discloses a technique in which the discharge fluid is introduced between the outer rotor and the rotor housing to reduce a resistance due to a sliding friction between the outer rotor and the rotor housing.
Further, in the gear-type pump, since the pressure at the discharge port side is high, the outer rotor is strongly pressed against a part of the rotor housing to produce a partial wear. Therefore, it is known, for example, from U.S. Pat. No. 4,820,138 to introduce the discharge fluid to a predetermined position at the discharge port side to produce a counter pressure against the high pressure at the discharge port.
However, in the gear-type pumps disclosed in Japanese Patent Unexamined Publication No. 60-17281 and U.S. Pat. No. 4,820,138, the inner rotor is positioned and rotated by a rotation shaft of the motor. There has been encountered a problem that the inner rotor is not accurately positioned due to a dislocation of the motor rotation shaft and a gap between the motor rotation shaft and its mating engagement portion. As a result, the gap between the inner rotor and the outer rotor increases, which has resulted in a problem that the liquid-tightness in the pump chamber is lowered, thus lowering a pumping efficiency. In order to overcome these problems, it can be considered to support the inner rotor by a support shaft separate from the motor rotation shaft. In such a case, however, since the inner rotor rotates in contact with the support shaft, the required drive torque is increased so as to overcome the friction between the inner rotor and the support shaft, which has resulted in a problem that the motor for driving the pump consumes an increased amount of electric power. Moreover, the wear develops in the inner rotor and the support shaft, and with the increase of the wear, the shaking of the inner rotor increases, so that it has been difficult to maintain a predetermined performance for a long period of time. A further problem is that since the inner rotor is held against the support shaft from the discharge port side by the pressure of the discharge port, the wear at the discharge port side of the inner rotor is particularly increased.